1. Field of the Invention
The present invention relates to a flat display screen anode having phosphors excited by electrons, for example, of microtip type. It more specifically relates to the biasing of phosphor elements of an anode provided with phosphor elements of different colors biased per color, for example, alternate strips of phosphor elements organized in combs.
2. Discussion of the Related Art
FIG. 1 very schematically shows the structure of a flat microtip screen of the type to which the present invention relates. This screen is comprised of two plates. A first plate 1, currently called the cathode plate, is arranged to face a second plate 2, currently called the anode plate. The two plates are spaced apart from each other by spacers 3 regularly distributed in the screen surface, and a vacuum is created in the area defined by the two plates and a peripheral sealing joint 4.
Cathode plate 1 includes electron generation elements and pixel selection elements (not shown) that may be organized in different ways, for example, as described in U.S. Pat. No. 4,940,916 of the Commissariat à l""Energie Atomique in the case of microtip screens. Anode plate 2 is, in the case of a color screen, provided with alternate strips of phosphor elements, each strip corresponding to a color (red, green, blue).
FIGS. 2A and 2B very schematically show a front view and a cross-section view of a portion of an anode plate. In FIG. 2B, the surface corresponding to the internal screen surface faces up. The anode includes, for example, alternate strips 4R, 4G, 4B of respectively red, green, blue phosphor elements. As illustrated in FIG. 2B, the strips of phosphor elements are arranged on corresponding conductive strips 5R, 5G, 5B generally organized in combs, all strips 5R being interconnected, as well as all strips 5G and all strips 5B. In certain cases, the phosphor elements are distributed in elementary patterns, each of which generally corresponds to a pixel (in fact, a sub-pixel of each color for a trichromatic screen). The xe2x80x9cpixelizedxe2x80x9d phosphor elements can then still be addressed by biasing electrodes in conductive strips (5G, 5B and 5R) such as described in relation with FIGS. 2A and 2B, but a specific mask is used to deposit the phosphor elements.
Two great categories of flat screens can be distinguished according to whether the user looks at the screen from the anode side or from the cathode side. In the first case, the light emitted by the phosphor elements propagates through the anode plate (downwards in FIG. 2B). The material of conductive strips 5R, 5G, 5B then is transparent, currently indium and tin oxide (ITO). In the second case, transparent electrodes 5R, 5B, 5G are replaced with opaque and preferably reflective electrodes, so that the largest possible part of the light emitted by phosphor elements 4R, 4G, 4B is sent back to the cathode once the phosphors have been excited by an electron bombardment. Electron generating plate 1 then is at least partially transparent and the observation is performed through this cathode plate.
In a color screen (or in a monochrome screen formed of two alternate sets of strips of phosphor elements of same color), the sets of strips (for example, blue, red, green) are often alternately positively biased with respect to cathode 1, so that the electrons extracted from the emissive elements (for example, the microtips) of a pixel of the cathode are alternately directed towards phosphor elements 4R, 4G, 4B facing each of the colors.
The selection control of the phosphor that is to be bombarded by the electrons imposes controlling, respectively, the biasing of the phosphor elements of the anode, color by color. Generally, the strips 5R, 5G, 5B supporting phosphor elements to be excited are biased under a voltage of several hundreds of volts with respect to the cathode, the other strips being at a zero potential. The choice of the values of the biasing potentials is linked to the characteristics of the phosphor elements and of the emissive means.
In some cases, the anode may, while being formed of several sets of phosphor elements or the like, not be switched by sets of strips. All strips are then biased to a same potential, at least for the duration of a display frame. The anode is then said to be unswitched.
The potential difference between the anode and the cathode is essentially due to the inter-electrode distance, that is, to the thickness of the internal space. A maximum potential difference is desired for reasons of display brightness, which results in searching the greatest possible inter-electrode distance. However, the structure of the inter-electrode space, that includes spacers 3 likely to create dark areas in the screen if their size is too large, prevents from increasing this inter-electrode distance.
The necessary trade off leads to choosing an anode-cathode voltage value that is critical from the point of view of electric arc formation. Destructive electric arcs can then be caused by the smallest dimensional irregularity of the distance separating an emissive means of the cathode from the phosphor elements of the anode. Such irregularities are, moreover, inevitable given the small dimensions and the techniques used to form the anode and the cathode.
On the cathode side, a resistive layer is provided in the case of microtip screens to receive the microtips and thus limit the formation of destructive short-circuits between the microtips and a control grid associated with the cathode.
Conversely, on the anode side, arcs may occur not only between the cathode plate and those of the anode phosphor elements that are biased to attract electrons emitted by the microtips, but also between two neighboring strips of phosphor elements, due to the potential difference between the two strips. In the case of a monochrome screen where the anode is formed of a conductive plane supporting phosphor elements of same color or in the case of an anode (color or monochrome) with several unswitched strips, the risk of arcs only exists between the anode and the cathode.
To limit the occurrence of such lateral arcs, it is currently provided to arrange, between anode strips 5B, 5R, 5G, interstitial strips 7 made of an insulating material (generally silicon oxide).
However, in practice, the efficiency of such insulating strips is limited for several reasons.
First, these strips are inoperative with respect to the forming of electric arcs between the anode and the cathode.
Further, and although this does not necessarily appear in FIGS. 2A and 2B in which the scales have not been respected, phosphor elements 4R, 4G, 4B significantly extend beyond the interstitial strips. Indeed, the thickness of the strips of phosphor elements is generally on the order of some ten xcexcm and the forming of silicon oxide insulating strips of such a thickness is, in practice, incompatible with the technologies used for manufacturing the anodes, so that the thickness of strips 7 generally is on the order of 1 to 2 xcexcm, their width being on the order of 10 to 20 xcexcm.
Further, during the deposition of the phosphor elements through a deposition mask, a slight misalignment of the mask may occur, so that a portion of conductive strips 5R, 5G, 5B or of the insulated areas becomes accessible once the screen is completed and thus favors the forming of arcs.
A first known solution to attempt to reduce the forming of arcs between the anode and the cathode is to provide, at the end of each conductive strip 5R, 5G, 5B, a resistor between the power supply line and the strip. As soon as a strong current appears in the strip, the resistor causes a voltage drop. As a result, the potential difference between the conductive strip and the cathode decreases and has the overvoltage generating the arc disappear.
A disadvantage of such a solution is that it does not protect from the forming of a lateral electric arc, that is, an arc between two neighboring strips 5R, 5G, 5B. A local current circulation may indeed occur between two strips, which is then not prevented by the end resistors.
Another disadvantage of using such resistors in series with the strips is that the resistors are generally made of ruthenium, the resistivity of which is stabilized by anneal. This anneal at high temperature (on the order of 600xc2x0 C.) necessary to stabilize the resistor raises problems of compatibility with the screen manufacturing process that requires, for the case where the conductive strips are made of aluminum in the case of a transparent anode, temperatures under 600xc2x0 C. Further, such a manufacturing method by anneal is difficult to control.
Another disadvantage of series resistors interposed between the anode conductive strips is that they form heating areas for the anode conductive tracks at the screen periphery.
A second known solution is described in French patent application Ser. No. 2,732,160. This solution consists of depositing the strips of phosphor elements on strongly resistive strips and bringing the biasing necessary to the phosphors by lateral biasing strips on either side of each resistive strip.
Even though this solution can provide satisfactory results, it requires a significant space between each strip of phosphor elements to house therein two biasing conductors respectively associated with two neighboring strips while sufficiently separating these biasing conductors from each other to maintain a necessary lateral insulation between them. Thus, this solution is, in practice, more particularly intended for screens of low resolution.
Conversely and as an example, for an anode plate in which the surface of each pixel is a square having a side of approximately 300 xcexcm, the anode strips each have a neighboring width, but under 100 xcexcm, and insulating strips 7 have a width on the order of some ten xcexcm. In such a case, the implementation of a solution of local protection by a resistive layer laterally surrounded by biasing strips cannot be envisaged due to the small distance between anode strips.
The present invention aims at overcoming the disadvantages of conventional techniques by providing a flat display screen anode that suppresses the risk of occurrence of an electric arc between the anode and the cathode plate, or between two neighboring strips of phosphor elements of the anode, without adversely affecting the screen brightness.
The present invention also aims at providing a solution that is compatible with conventional distances between two strips of phosphor elements.
The present invention also aims at providing a solution that is particularly adapted to a xe2x80x9ctransparentxe2x80x9d cathode screen, that is, the cathode plate of which forms the screen display surface.
The present invention further aims at providing a solution that respects conventional anode manufacturing methods and, in particular, the masks used in this manufacturing.
To achieve these objects, the present invention provides a flat display screen anode, including phosphor elements intended for being excited by an electron bombarding, these elements being deposited on at least one biasing electrode including, at least under the phosphor elements, a resistive layer deposited on a conductive layer for biasing the phosphor elements.
According to an embodiment of the present invention, the phosphor elements are directly deposited on the resistive layer.
According to an embodiment of the present invention, the phosphor elements are deposited on a reflective conductive layer, itself deposited on the resistive layer.
According to an embodiment of the present invention, the reflective layer is deposited according to elementary patterns of small dimension in the anode surface.
According to an embodiment of the present invention, the phosphor elements are deposited according to the elementary pattern of deposition of the reflective layer.
According to an embodiment of the present invention, the resistive layer is not patterned.
According to an embodiment of the present invention, the resistive layer has the same pattern as the reflective layer.
According to an embodiment of the present invention, the resistive layer has, at least in the active screen area, the same pattern as the biasing conductive layer.
According to an embodiment of the present invention, said conductive layer has a pattern of alternate strips interconnected in at least two sets.
The present invention a so provides a flat display screen including a cathode generating electrons bombarding a cathodoluminescent anode as mentioned above.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in conjunction with the accompanying drawings.